There are two types of xDSL (i.e. ADSL, VDSL, etc.) installations that are typically employed at a customer premises. In one, the telephone line connects to a splitter as the line enters the house. Leaving the splitter are two lines, one of which connects to the xDSL modem, the other of which connects to all of the telephone devices in the house, typically using the existing in-home wiring. The splitter separates the low frequency voice band telephone signal from the higher frequency xDSL signal.
In the other typical type of deployment, a splitter is not used. Instead, the combined voice band and xDSL signals are routed throughout the house over the existing telephone wiring. The signals are able to share the same wiring due to the use of different frequency bands. In this case, a filter, known as a micro-filter or a micro-data filter, is connected in-line with each telephone device.
The problem of xDSL performance being degraded by a missing micro-filter is a long standing problem that has been around since the early days of ADSL and continues with VDSL. In splitter-less installations, it's possible that the xDSL modem can be installed by the user. In these cases, the user must also properly connect a micro-filter in-line with each telephone device which is connected to the same telephone line within the building. This must be done not only for each actual telephone, but also any device that connects to the telephone line such as an analog modem, fax machine, or alarm system. It is quite common for the user to fail to connect a micro-filter to one or more of these devices and to be unaware that they have failed to do so.
When the micro-filter is not installed it causes several problems. When the unfiltered xDSL signal reaches the telephone device the total voltage of the broadband signal can be greater than that of a narrow band voice signal, so that the voltage exceeds the design limits of the phone's line interface circuitry. This can cause distortion. This distortion impacts the modem's performance by lowering the data rate (i.e. capacity).
Another problem is that an unfiltered phone affects the impedance seen by the modem. Worse yet, when the phone's hook status changes (on-to-off or off-to-on), the impedance changes. This modifies the echo seen by the modem's receiver and can cause the line to drop when the modem is in Showtime.
Yet another problem associated with missing micro-filters is that for most types of phones, the amount of distortion generated depends on whether the telephone device is on-hook or off-hook. The problem is that at the time a Single Ended Line Test (SELT) is performed, a phone which has a missing micro-filter might not be in its highest distortion state and therefore the distortion might not be detectable.
What is needed, therefore, is a means of detecting a missing micro-filter that addresses these and other problems.